“People learn, early in their lives, what is their reason for being,” said the old man, with a certain bitterness. “Maybe that’s why they give up on it so early, too. But that’s the way it is.” - The Alchemist, Paulo Coelho

The Rule of One for copywriting:

You have one reader. One big idea. One promise. And one offer.

Source: unknown person who is no longer alive, probably in advertising and direct response copywriting in the 1920’s.